The present invention relates to an image processing apparatus and image processing method and, more particularly, to an image processing apparatus and image processing method for representing a multivalued image inputted by e.g., a scanner or computer or the like, in pseudo tones when the multivalued image is repeatedly bitmapped to be outputted to a device having a low tonality-representation capability.
Error diffusion method and dither method are known as the conventional method of pseudo-tonality representation. According to the error diffusion method, as shown in FIG. 10, a diffusion coefficient is assigned to plural subsequent pixels of a pixel of interest, so as to distribute a quantization error generated at the pixel of interest to the neighboring pixels in accordance with the diffusion coefficient. By this processing, density values of the entire image is preserved, enabling excellent pseudo-tonality representation.
According to the dither method, as shown in FIG. 11, a dither matrix with a given threshold value is prepared (in FIG. 11, the given matrix size is 4.times.4 pixels), and each pixel of the inputted multivalued image data is compared with the matrix on a one-to-one basis. If a pixel value of the inputted multivalued image data is larger than or equal to the threshold value, the pixel is set to ON, and if a pixel value of the multivalued image data is smaller than the threshold value, the pixel is set to OFF. As a result, an output having binary values is obtained. According to the dither method, since the regular arrangement of dots produces an artificial pattern, the resultant pseudo-tonality image has graininess. Therefore, an image reproduced by the dither method tends to have lower quality compared to an image reproduced by the error diffusion method.
Herein, taking a repeated design pattern as an example, such as designs printed on a fabric, e.g., a tie or handkerchief or the like, there are patterns as shown in FIGS. 12A to 12C: a simple pattern (FIG. 12B) generated by repeatedly printing a basic pattern (FIG. 12A), which is the base of the repeated design pattern, simply in horizontal and vertical directions; a zig-zag pattern (FIG. 12C) generated by repeatedly printing the basic pattern in horizontal and vertical directions while alternately shifting the basic pattern by half the size of the basic pattern, and the like. However, in a case where the basic pattern must be repeatedly printed for a large number of times, it is inefficient to repeatedly perform pseudo-tone processing on the entire image. In addition, if this processing is to be performed by executing software, the processing becomes unrealistic in view of its processing speed.
Furthermore, if only the basic pattern is binarized and the binary data is simply repeated to form an image, conspicuous noise is generated at the boundary portion of the basic patterns (this noise will be referred to as boundary noise) as shown in FIG. 13. It is considered that the noise is caused by discontinuity of binary processing at the boundary portion.
The suggested methods to eliminate the boundary noise include: a method where an exterior frame is added to the basic pattern to be drawn as shown in FIG. 14A (hereinafter referred to as "exterior-frame-addition method"), or a method where smoothing is performed at the boundary portion of the basic pattern as shown in FIG. 14B (hereinafter referred to as "boundary-smoothing method"). In the exterior-frame-addition method, the same multivalued image having a predetermined size is added to surround the basic pattern, and binary processing is performed on the entire image. As a result, binary data of the basic pattern is obtained. However, even if the obtained binary data is used repeatedly to form an image, boundary noise still appears due to the discontinuity at the boundary portion.
Moreover, even if smoothing processing is performed at the boundary portion of the binary data to eliminate the boundary noise of the obtained binary data by employing the boundary smoothing method, this still causes a problem of making the boundary portion unclear in a case where, for instance, the image of the boundary portion includes an edge portion such as thin lines.